What GPS can I buy that I could use both in car and on the golf course?

My suggestion is to buy separate devices for each use since the golf caddie (sky caddie) is specifically designed for a golf course and downloadable software for several courses and the navigation GPS for travel (auto) has maps and destination points specific for road navigation. I suggest a Garmin GPS for the auto as they have several handheld and auto mountable models to choose from as well as good customer support and updates. As for the golf GPS I suggest the Sky Caddie SG4 model since it is waterproof and has an SD slot for programmable updates and other options. Plan on spending $350.00 + for each as there's no cheap way out on this if you want quality and reliability. No one likes to be lost or stuck in traffic if they have a GPS that isn't functioning properly or doesn't have the features available because you went the inexpensive route.

To be useful on the golf course as well as do the the car GPS navigation I'd suggest teaming a GPS enabled PDA such as the HP iPAQ hw6515 (http://gpstekreviews.com/category/gps-pdas/) with the appropriate software - eg. Microsoft Pocket Streets (http://www.microsoft.com/windowsmobile/mobilelife/default.mspx)for navigation and Personal GPS Golf System (http://www.golfps.com/) for the golf course.